1. Field of the Invention
The present invention relates to a coating apparatus for coating a surface of each of objects to be coated, which are supplied one by one, with a liquid paint by a die coater, for example, in precision coating works such as coating of a resist in the field of fabrication of semiconductors, coating of an ultraviolet absorbing layer in the filed of fabrication of optical filters, and so on, a coating method, a method for producing a color filter substrate, and a liquid-crystal display apparatus using the color filter substrate produced by the method.
2. Related Background Art
There have been strong demands for the technology of thinly and uniformly coating small substrates having the length less than 1 m in the coating direction, such as plastic substrates for optical filters, glass substrates for liquid-crystal displays, glass substrates for color filters, and so on, with a variety of paints. For industrially forming a coating film on such substrates, a single-substrate coating method is employed; the objects to be coated (coated objects) are supplied one by one to a coater to be coated with a paint and be conveyed to the next step such as drying or the like.
Spin coaters, bar coaters, and roller coaters are commonly used for forming the coating film on the coated objects.
Among them, a method using the spin coater is a method widely used in coating of photoresist on semiconductor wafers, in which the coating film can be formed by dropping the paint onto the center of the surface of the rotating object to be coated. Thicknesses of coating films obtained by this method can be made uniform considerably accurately throughout the entire region of the coated object by selecting the paint from those of kinds suitable for this method. The method, however, requires an extremely large use amount of the paint in order to yield the coating film in the desired thickness and thus is not economical. In addition, the paint sometimes attaches to the edge and the back surface of the coated object and the paint scattered into the apparatus can be gelled or hardened therein, so as to degrade stability and cleanliness of steps, thus causing degradation of quality of coated products.
A method using the roller coater is a method for transferring the paint onto the coated object via a roller, which can be applied to coating of long coated objects and to coating of coated objects rolled in a roll form. However, since the paint is fed successively from a pan via the application roller onto the coated object in this method, the paint is exposed to air for a long period, so that the paint is apt to absorb moisture and deteriorate because of oxidization. In addition, mixing of foreign matter into the paint is also easy to occur. These will result in degrading the quality of coated products.
A method using the bar coater is a method for coating the coated object with the paint by use of a bar in which a thin wire is wound around a rod. This method has the disadvantage of stripes easily appear in the coating film, because the wire around the rod contacts the coated object.
Taking such disadvantages into consideration, a die coating method using the die coater has been proposed recently. Proposals of application of the die coater to the production of color filters are present in Japanese Patent Applications Laid-Open No. 5-11105, No. 5-142407, and No. 6-339656.
The die coaters have been adopted popularly heretofore in use of coating of thick films and continuous coating with high-viscosity paints, and coating methods known for forming the coating film on the coated object by the die coater include a curtain flow method, an extrusion method, a bead method, and so on, as described in U.S. Pat. Nos. 4,230,793, 4,696,885, and 2,761,791. Among them, the above bead method forms the coating film in such a manner that the paint is delivered through a slit provided in a mouthpiece of the die coater to form a paint mass called a paint bead between the mouthpiece and the coated object moving relative to the mouthpiece while maintaining a constant clearance thereto and that the paint is drawn out in this state with movement of the coated object to form the coating film. By adopting the bead method for continuously forming the coating film while supplying the same amount of paint through the slit as an amount of consumption for formation of the coating film, the uniformity of thickness can be accomplished with considerably high accuracy in the coating film thus formed. Since there is little waste of the paint and since a paint feed path is hermetically closed before the delivery through the slit, this method can prevent the deterioration of the paint and the mixing of foreign matter and can maintain the quality of resultant coating film high.
However, in cases wherein the die coater is employed for coating of a color layer in the production of the color filters for liquid-crystal displays, nonuniformity of thickness directly affects the quality in the form of nonuniformity of colors and also affects the gap of the liquid-crystal layer in coating of overcoat layer. There is thus the demand for coating of thin films with high uniformity of thickness.
An area that can be used as an effective region is a region in which the uniformity of thickness is within a fixed value. Since the die coating method is a discontinuous single-substrate coating method, it inevitably forms rises of nonuniform thicknesses at the coating start and end portions. For that reason, the effective region by the die coating method tends to become smaller because of the rises than that by the spin coating methods of the prior art.
The rises have the highest part at least 1.5 times greater than the average film thickness of the coating surface, and thus a problem can arise in steps after the coating step, for example, in development or the like of the photoresist after the coating in certain cases.
Methods for solving the problem of the above rises are normally methods of controlling the relative speed of the coated object to a paint dispensing device, a dispensing rate of paint (a dispensing amount per unit time), the distance (clearance) between the substrate and the tip of the mouthpiece of the paint dispensing device, and so on. Further, there are also suggestions to solve this problem by controlling the shape of the paint bead, as described in Japanese Patent Application Laid-Open No. 8-229497. However, where the film thickness is controlled during the period from the coating start to a steady coating state or during the period from the steady coating state to the coating end by controlling the above relative speed or dispensing rate, it is not easy to perfectly control behaviors of the paint bead varying instantaneously. When the thickness is adjusted by a method for decreasing the film thickness, it can induce a break of the paint bead and in turn degrade the stability of the coating step. From the viewpoint of the stability of the coating step, it is desirable to keep the volume of the paint bead at some large value.
Many films formed by coating in the production of color filters for liquid-crystal displays are thin films having the film thickness of not more than 10 .mu.m in a dry state. For forming such thinner coating films, the volume of the paint bead becomes smaller. When the volume of the paint bead is small, the paint bead will be broken with failing to be retained continuously in the coating width direction unless sufficient adsorption force is obtained between the paint and the surface of the coated object. As a result, there will appear a defect of stripes in the coating film.
There is such a general tendency that the die coating methods have a narrower permissible range for surface energy than the spin coating methods. Therefore, the die coating methods are apt to cause a defect in the case of coating of a thinner coating film or in the case of coating with a paint of low surface energy.